Abstract: A catalyst for polymerising 1-olefins, comprising (a) a tetradentate ligand I and II as illustrated in the specification wherein; D and D? are phosphorus or nitrogen; Q and Q are bridging groups forming part of a ring; B is a bridging group between D and D?; R1 and R9 are each independently a polar group or phenyl, naphthyl, anthryl, phenanthryl, triptycyl or a heteroaromatic ring; R5 to R8 are selected from hydrogen, halogen, hydrocarbyl, heterohydrocarbyl, NR?2, PR?2, OR?, SR? or SiR?3 where each R? is independently selected from hydrogen, halogen, hydrocarbyl, heterohydrocarbyl, and any adjacent groups may be joined together to form a ring; in the case I, A and A? are independently OH, 0?, SH, S?, NR?H, R?N?, PR?H or R?P?; and in the case II A and A? are independently NH, N?, PH or P?, where R? is defined as for groups R5 to R9 above; and R5 and R5?, R6 and R6?, or R7 and R8 may be joined together to form a ring; (b) a source of Group 3 to 10 transition metal or a lanthanide metal and optionally (c) an act